Chain tensioner on Brompton not spring-loaded anymore?

I took apart the chain tensioner/derailleur on my Brompton today for a full cleaning. When I went to put the tensioner arm back on, the spring no longer loaded--it just kind of sat there. I made sure the ends of the spring were inserted into the little slots in the plastic housing, checked to be sure the bolt wasn't on too tight...still nothing. So now I have no chain tension at all. How the heck do I get this thing back on so it's spring-loaded?! I have no idea what I did wrong.

Naturally, the LBS that services my Brompton is closed until the new year, so if I really broke something then I'm stuck until then.